Huh - giving a (sort of) answer to my own question - saw the solution on KVR - maybe for somebody else who needs the same:
there is a Halion database - well two databases, one for H6, one for HS3. Location (on Windows) is: C:\Users\[username]\AppData\Roaming\Steinberg\Halion Sonic 3_64
C:\Users\[username]\AppData\Roaming\Steinberg\Halion 6_64
Database file name is the same for both: mediabay3.db
I made a symbiotic link for Halion Sonic DB to be linked on Halion DB; so both uses the same database, and changes are preserved in both ways (you can make a change in either HS or H, and it will be the same on the other one).
Procedure:
Backup your mediabay3 databases, both of them, just in case...
Delete (yes, delete) the mediabay3.db file for HALION SONIC 3
Run as Administrator Command Prompt, and put the following:
mklink "C:\Users\[username]\AppData\Roaming\Steinberg\Halion Sonic 3_64\mediabay3.db" "C:\Users\[username]\AppData\Roaming\Steinberg\Halion 6_64\mediabay3.db"
Of course, you need to change [username] with your username, without brackets
